Hydroprocessing catalysts usually comprise one or more sulfided Group 6 metals combined with one or more Group 8 to 10 metal promoters on a refractory support, such as alumina. Bulk, unsupported catalysts are also known. Hydroprocessing catalysts that are particularly suitable for hydrodesulfurization, as well as hydrodenitrogenation, generally comprise molybdenum and/or tungsten sulfide promoted with a metal such as cobalt, nickel, iron, or a combination thereof. These sulfided catalysts generally have a layered or platelet morphology.
Current research into hydroprocessing catalysts is being driven by the need to produce distillate fuels with lower levels of sulfur and nitrogen, as mandated by environmental regulations, while at the same time meeting the needs of refiners to process crude oils with larger amounts of these heteroatoms. A significant need therefore exists to find catalysts which can do more efficient desulfurization and denitrogenation, particularly when existing hydroprocessing units are limited in their pressure capability.
The ability to modify the nanostructural morphology of hydroprocessing catalysts provides a possible way to control their activity and selectivity. Thus, in U.S. Pat. No. 7,591,942, it was demonstrated that sulfiding a bulk bimetallic Ni (or Co)/Mo (or W) phase containing a surfactant amine with a backbone containing at least 10 carbon atoms gave a catalyst comprising stacked layers of MoS2 (or WS2) having a reduced stack height as compared to that obtained by sulfiding the carbon-free bulk oxide. A similar result was reported for bulk ternary Ni—Mo—W catalysts in U.S. Pat. No. 7,544,632. Lower stack heights are important, since they imply the presence of smaller crystals of Mo/W sulfides, which in turn results in a larger surface area available for catalysis.
Another potential route for controlling catalyst activity is the generation of lattice defects in the crystal structure of the catalyst since lattice defects can create special sites associated with increased activity and/or selectivity. See Kaszstelan, S. A. “Descriptive Model of Surface Sites on MoS2 (WS2) Particles,” Langmuir, 6 (1990), pages 590-595.
Recent work by the present inventors using ex-situ transmission electron microscopy (TEM)-based time-temperature-transformation sulfidation studies has shown that ditrital NixS particles develop before MoS2/WS2 during sulfiding of molybdenum and tungsten oxides. Conventional TEM (CTEM) imaging, in combination with elemental analysis via energy dispersive spectrometry (EDS) and TEM tomography (TEMT), reveals that MoS2/WS2 particles grow as relatively straight layered structures in regions where no detrital NixS particles are detected. Thus, these relatively straight layers of MoS2/WS2 require minimal, if any, lattice defects to form. However, during sulfidation, hydrogen spillover at detrital NixS particle surfaces results in nucleation and growth of layered MoS2/WS2 structures with a curved morphology. CTEM, EDS, and TEMT data indicate that the MoS2/WS2 particle curvature conforms to that of the detrital NixS particle nucleating surface. Thus, these MoS2/WS2 structures develop lattice defects to accommodate their growth around the detrital NixS particle's surface. Because lattice defects can create special sites associated with increased activity and/or selectivity, the ability to control defects sites and their site density is important.
Thus, according the present invention, it has been found that by “seeding” a Mo(W) oxide precursor material with size and shape-controlled NixS particles, the NixS seeds can control the curvature of the MoS2/WS2 particles produced on subsequent sulfiding, and hence the MoS2/WS2 defect sites and defect site density. Similar MoS2/WS2 morphological control can be achieved in systems seeded with CoxS particles or containing other Group 8 to 10 metals that sulfide at temperatures lower than Mo(W). The present “seeding” phenomenon seems similar to the templating action exhibited by various organic materials used to direct the structure of various zeolitic materials. However, it is believed that the concept of using an inorganic phase as the templating agent for another inorganic phase has never been previously been documented.